Default disk partitions

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When I installed Ubuntu 12.10, I installed it onto a 500 GB hard drive using the default partitioning.
Looking at the hard drive in Disks
I see the main partition (/dev/sda1 = 496 GB) and the swap partition (/dev/sda5 = 4.3 GB). I also see another partition (/dev/sda2 = 4.3 GB), which is not mounted. Its type is Extended and its contents are Extended Partition.
Here is the output from ls /dev/sda*:
/dev/sda /dev/sda1 /dev/sda2 /dev/sda5
Why aren't there partitions 3 & 4? And what is /dev/sda2 for?
(HowTos)